1. Field of the Invention
The present invention relates to a process for refining molten steel using a ladle refining furnace and, more particularly, to a process for refining molten steel to obtain low-nitrogen steel or low-phosphorus steel.
2. Description of the Prior Art
In a conventional process for refining molten steel, blowing of molten steel is performed in a converter for refining. After the temperature is raised to about 1700.degree. C., the steel is tapped into a ladle, and a deoxidizing agent and a ferro alloy are charged into the steel upon tapping. The molten steel in the ladle is bubbled in the presence of slag so as to adjust the composition of the steel.
When the O.sub.2 concentration in molten steel decreases, the N.sub.2 adsorption capacity of molten steel increases. In the above-mentioned conventional refining process, the N.sub.2 adsorption capacity of the molten steel is increased since deoxidation is performed during tapping. This causes inclusion of N.sub.2 into the molten steel or pick-up of N.sub.2, thereby increasing the N.sub.2 concentration in the molten steel.
Aluminum as a deoxidizing agent partially reacts with slag, lessening its contribution to deoxidation. In view of this, aluminum must be added in an excess amount in consideration of the fraction which reacts with slag. In association with this problem, the amount of aluminum which reacts with slag changes in each refining process. For this reason, even if aluminum is added in a predetermined amount, the deoxidation amount varies in each refining process and desired deoxidation cannot be performed.
The phosphorus concentration in the molten steel upon tapping is proportional to the tapping temperature of the molten steel. FIG. 1 shows a relationship between the tapping temperature and the phosphorus concentration in molten steel after blowing. In the conventional refining process, the phosphorus concentration increases since the tapping temperature is as high as about 1,700.degree. C. Bubbling in a ladle is performed for deoxidized molten steel and in the presence of slag. Therefore, phosphorus in the slag causes rephosphorization of molten steel, and the phosphorus concentration increases. In the conventional refining process, phosphorus concentration can be reduced to only about 150 ppm.